Wetting indicator for composites

ABSTRACT

The invention relates to a device for obtaining, in the manufacture of a high-strength composite, an indication the reinforcement layer of the composite has been wetted throughout by the matrix material of the composite. The reinforcement layer is of the type which mainly consists of fibers or fiber bundles which do not become translucent or transparent when wetted by the matrix material, such as carbon or aramide fibers or fiber bundles. The matrix material of thermosetting plastic is applied in a liquid state in the manufacture according to, for instance, the technique of hand lay-up. The reinforcement layer includes indicating fibers which become translucent or transparent when wetted by the matrix material. A good indication is obtained if the indicating fibers have such properties that they become translucent only when the adjoining reinforcing material has been completely wetted throughout. Preferably the indicating fibres are uniformly distributed across the reinforcement layer and in an amount that does not affect the properties of the composite significantly.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The invention relates to a device for indicating that a reinforcingmaterial, which is made of fibers and included in a composite materialand which does not become translucent or transparent in wetting, hasbecome wetted throughout by the matrix of the composite material whilemaking the composite material. The device can indicate, for instance,that the carbon fiber reinforcement for the material of a hull or thelike has been completely wetted throughout by liquid thermosettingplastic in the manufacture of the hull.

2. Description of the Related Art

An object of a composite material can be made by the material, whichcontains a reinforcement and a matrix that is liquid at certain stagesof the manufacture, being supplied to a mould where the object obtainsits shape when the matrix sets. Depending on whether the mould is of thetype having a cavity or only a surface determining the shape of theobject, the shape will be substantially determined by all its surfaceportions or only the inside or outside of the object. When manufacturinge.g. hulls of reinforced thermosetting plastic, moulds determining onlythe outer surface of the hull, so-called female moulds, are used in mostcases.

One technique of manufacturing hulls is laminating by hand lay-up. Thefibre reinforcement consisting of cut-out parts of a fabric or mat isarranged on a layer of liquid plastic which has been rolled on with aroller onto the mould surface. The reinforcement is then worked with asoft roller or brush so as to be impregnated and wetted with plastic.Finally, air and excess plastic are pressed out by means of a metalroller from the layer and the space between the layer and the underlyingsurface. Moreover, minor inclusions of air that are bound to thesurfaces of the reinforcing fibres are removed. Owing to such working,wetting is further improved. The entire surface of each fiber of thereinforcement will then be substantially completely coated with matrixmaterial.

To this first layer, additional layers are applied, as described abovefor the first layer. For large objects, use can be made of intermediatecurings when making the laminate. Otherwise, all the layers can beapplied in succession, curing taking place after application of the lastlayer.

After the application of each layer, it is necessary to make sure thatthe reinforcement of the layer has been properly wetted throughout. Ifthe composite material should contain inclusions of air or voids, thehull will have an inferior quality with a risk of inferior strengthsince the composite can delaminate adjacent the inclusions of air andwater can diffuse in the inclusions and cause chemical degradation ofthe material and cracking by frost. Insufficient wetting also reducesthe resistance to compression of the composite, which may cause breakingof the hull.

For high-strength composite materials, a reinforcement consisting ofpractically merely carbon fibres or aramide fibers or a mixture thereofhas recently come into use. The drawback of these materials is that itis difficult to visually form an opinion whether they have become wellwetted throughout. To make sure that the wetting has been sufficient,ultrasonic testing and other testing techniques have been applied, whichhas contributed much to these composites being very expensive. The factthat the manufacture must be particularly precise owing to the lack ofeasy checking has also contributed to the increased cost.

BRIEF SUMMARY OF THE INVENTION

The object of the invention is to provide a device as mentioned in theintroduction and having the following properties:

When manufacturing composite materials, it should be easy to make surewhether a reinforcing material has been well wetted throughout. Thisapplies to reinforcing fibres in e.g. high-strength composite materials,the wetting of which has previously been difficult to estimate.

It should be possible to carry out the inspection in a short time andwithout expensive and complicated equipment.

The object is achieved by the device having the features as defined inthe appended claims.

According to the invention, it is suggested that, intermixed with thereinforcing material, elements are added which have the same wettingproperties as the reinforcing material, preferably as fibers in the formof threads, bundles, flakes, wads or the like of a material whichbecomes translucent or transparent by a shade of colour that depends onthe nature of the material when its surface has been wetted by thematrix material. This indicating material, which preferably should alsoserve as reinforcement, should be added in an amount that is relativelysmall, such that the properties of the composite material are notchanged to an unacceptable extent, but yet sufficient to obtain a goodindication. The amount, which in the preferred embodiment is selected tobe 4%, should in normal applications be less than about 5%. The elementsadded should be uniformly spread in the reinforcing material in such amanner that if they have all become wetted throughout, it can be assumedthat the reinforcing material in the areas round the elements has alsobecome wetted. The distances between the elements should not be chosento be greater than to make these surrounding areas together cover theentire reinforcing material. Mixed reinforcements containing e.g. carbonfiber and glass fibre materials are already available, but the purposeof these reinforcements is to obtain other properties or a lessexpensive reinforcement. The admixture of glass fibers therefore is muchgreater, about 50%, as compared with reinforcements according to thepresent invention.

The reliability of the indication will be improved if the elements havesuch properties that they will be translucent/transparent only when theadjoining reinforcing material has become completely wetted throughout.These properties can be affected by the choice of the material of theelements and surface treatment.

When the composite material is a reinforced thermosetting plasticintended for hulls, use is often made of ester plastic or epoxy plasticas matrix material. In their liquid state, these materials aretransparent in ordinary light. The indicating elements may then consistof certain polymers or glass fiber materials which from being, forinstance, white in dry state become translucent or transparent whenwetted. They will then let through the colour of the base, which can bethe colour of the gel coat or the black colour of the subjacent carbonfibre reinforcement. Of course, a colour of the elements should beselected that is different from that of the base in order to obtain aclear change of colour. For other matrix and indicator materials, it maybe necessary to use a light of a different wave range, for instance, thefluorescence from elements that have become wetted can be used forindicating purposes.

Reinforcing materials of carbon fibers often are in the form of a fabricor a stiched so-called multiaxial mat. The warp and weft threads of thefabric consist of flat bundles of a large number of carbon fibers orfibrils. The mat, which is used if a more rigid composite having ahigher fiber content is desired, may consist of two plies, eachcomprising parallel threads of the same type as those used in thefabric. The threads of the various plies are allowed to be oriented indifferent directions. When two plies are used, the threads of the secondply are often directed perpendicular to the threads of the first ply.For fixing the threads, seams are made through the plies. Areinforcement of aramide fibers is carried out in a similar manner.

According to the invention, it is further suggested that a fabric or matcomprises, as elements, threads or bundles of the above-mentionedmaterial, which is suitable for indicating. By using such threads of acolour contrasting with that of the reinforcing material, such as abright colour in the black carbon fiber material, they will be easy tospot in inspection. The threads, which can have the same thickness asthe threads of the reinforcing material, are inserted preferably at thesame distance from each other in the warp and in the weft as well as inboth plies or all plies. This distance should give the elements aspreading in the reinforcing material that, as stated above, isdetermined by the used materials, the type of composite, themanufacturing technique etc.

When indicating threads, which become translucent or transparent whenwetted, are inserted in both the warp and the weft and in the two pliesof the mat, spots arise where the threads intersect, i.e. parts of thebuilt-up layer which, when wetted, become translucent or transparentthroughout the layer. If more than two plies are used in a mat, theindicating threads are arranged in the additional plies, such that theyextend in the above-mentioned spots or restricted parts. In this manner,indication is obtained throughout the layer. As a result, it is possibleto make sure that the layer has been wetted throughout its thickness,not only in its uppermost ply.

If the layers include a reinforcing fabric or mat having threads orbundles in one ply only, the reinforcement is usually laid with thethreads oriented in different directions in the layers. In theintersection of indicating threads, spots for indicating in two layerswill be obtained.

The invention is not restricted to manufacture by simple hand lay-up. Itcan also be used in vacuum infusion using a transparent coating foil andfor preimpregnated reinforcing materials, the wetting being checkedbefore manufacturing the composite object.

According to the invention, it is possible to have a quality control ofe.g. high-strength composite materials with reinforcement of carbonfibers and aramides. The appearance of these materials does not changewhen wetted, and therefore it has previously been necessary to usetime-consuming and expensive techniques.

The device according to the invention causes an indication in depththroughout the layer last applied, in some cases throughout thethickness of the entire applied material.

The inspection can be carried out more systematically since a checkedpattern or the like is available for assistance.

It is possible to avoid expensive preparation of the carbon fibermaterial, which is often regarded as necessary for complete wetting,such as preimpregnation and subsequent cold storage to prevent thecuring of the impregnation.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment will be further described with reference to theaccompanying drawings.

FIG. 1 shows a reinforcing material in the form of a fabric containingelements according to the invention.

FIG. 2 shows a reinforcing material in the form of a two-ply mat.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a portion of a plain weave fabric 1 to be used asreinforcement in a composite. The fabric, which is of standard type withthe exception of the inserted indicating elements which will bedescribed below, comprises bundles of the same type for both the warp 2and the weft 3 of the fabric. The bundles are of an ordinary kind andare elongate in cross-section having a height which is small in relationto the length and consist of a large number of parallel longitudinalfibers of fibrils of carbon fiber. Instead of every twenty-fifth carbonfiber bundle in warp as well as weft, indicating elements 4 in the formof bundles of glass fibers are inserted. In the preferred embodiment,these bundles are of a cross-section corresponding to that of the carbonfiber bundles so as to prevent the thickness of the fabric from beingirregular. The ratio of 4% of the amount of indicating material, glassfibers, to the amount of reinforcing material, carbon fibers and glassfibers, has been selected empirically. In other types of fabrics, otherratios can be selected. If narrower bundles of glass fibers are used,the ratio can, of course, be reduced to, for instance, 2.5% which inrespect of strength is more favourable, or the distance between theglass fiber bundles can be reduced at the same ratio. This ratio shouldnot be so great that the reinforcement is impaired in an impermissiblemanner. Also the manufacturing technique is of importance. If, forinstance, during application, the layers are worked more between thebundles than on the bundles, the ratio can be decreased without the riskof error in indication being unacceptable.

FIG. 2 illustrates a reinforcing material of reinforcing fibers, such ascarbon fibers or aramide fibers, in the form of a multiaxial mat. Themat, as illustrated, consists of two plies generally designated by thereference numerals 5 and 6. Each ply comprises parallel threads of thesame type as those used in the fabric. The threads 7 of the first ply 5are oriented in a different direction from the threads 8 of the secondply 6. The threads 7 and 8 consist of flat bundles of a large number offibers or fibrils. When two plies are used, the flat bundles or threads8 of the second ply 6 are often directed perpendicular to the threads 7of the first ply 5. For fixing the threads, seams can be made throughthe plies.

Interspersed and aligned with the threads 7 and 8 are similar threads orbundles of indicating fibers 9 and 10, respectively. The indicatingthreads are of a color contrasting with that of the reinforcingmaterial, such as a bright color in the black of a carbon fibermaterial, so that the indicating fibers will be easy to spot uponinspection. The indicating bundles 9 and 10, which can have the samethickness as the bundles 7 and 8 of the reinforcing material, areinserted preferably at the same distance from each other in both plies,if two, or in all plies if more than two. This distance should give theelements a spreading in the reinforcing material that, as statedpreviously, is determined by the used materials, the type of composite,the manufacturing technique, etc. With the indicating threads insertedin both plies of the mat, spots 11 arise where the threads 9 and 10intersect upon becoming translucent or transparent when wetted.

The manufacture of composite is carried out in the same manner whetherthe reinforcing material comprises elements according to the inventionor not. However, the inspection after application of each layer iseffected more systematically by making sure that all the fiber glassthreads and especially their intersections have changed from theoriginal colour to a colour which depends on, inter alia, the base ofthe previously applied layer.

I claim:
 1. A composite containing a matrix material of a thermosettingplastic and a reinforcement layer, the reinforcement layer having aplurality of elongated reinforcing fibers, wherein said reinforcingfibers do not become translucent when wetted by said matrix material,said reinforcing fibers including at least a first series of fibers anda second series of fibers, said reinforcing fibers of said first seriesof fibers being generally aligned in a first direction, said reinforcingfibers of said second series of fibers being generally aligned in asecond direction different from said first direction so that said firstseries of fibers crosses said second series of fibers, wherein theimprovement comprises:a plurality of indicating fibers, at least one ofsaid indicating fibers being generally aligned and substantially planarwith said reinforcing fibers of at least one of said first and secondseries of fibers, said indicating fibers becoming translucent whenwetted by the matrix material, said indicating fibers being present insuch small quantity that the properties of the composite are notsignificantly different from the properties of a composite containingonly reinforcing fibers.
 2. The composite as claimed in claim 1, whereinthe indicating fibers are uniformly distributed across the reinforcementlayer.
 3. The composite as claimed in claim 1, wherein the indicatingfibers are made of a reinforcement material, so that the indicatingfibers reinforce the composite.
 4. The composite as claimed in claim 3,wherein the reinforcement material of the indicating fibers is selectedfrom the group consisting of glass and polymer.
 5. The composite asclaimed in claim 1, where in the volume of the indicating fibers is amaximum of about 5% of the total volume of said reinforcing fibers andsaid indicating fibers.
 6. The composite as claimed in claim 5, whereinthe volume of the indicating fibers is about 4% of the total volume ofthe reinforcing fibers and the indicating fibers.
 7. The composite asclaimed in claim 1, wherein the reinforcement layer includes a fabric,the first series of fibers is a warp, the second series of fibers is aweft, and the warp and weft interweave to form the fabric.
 8. Thecomposite as claimed in claim 7, wherein said at least one indicatingfiber is included in the warp of the fabric, and at least one additionalindicating fiber of said indicating fibers is included in the weft ofthe fabric, andthat the fabric is so arranged that the at least one andthe at least one additional indicating fibers form an intersectionwhich, when wetted, becomes translucent throughout the fabric.
 9. Thecomposite as claimed in claim 1, wherein the reinforcement layerincludes a mat, and each of the first and second series of fibers is amat ply.
 10. The composite as claimed in claim 9, wherein the at leastone indicating fiber is inserted in one of the mat plies, and at leastone additional indicating fiber of said indicating fibers is inserted inthe other of the mat plies, wherein the at least one and the at leastone additional indicating fibers form an intersection which, whenwetted, becomes translucent throughout the mat.
 11. The composite asclaimed in claim 1, wherein the indicating fibers become transparentwhen wetted by the matrix material.
 12. The composite as claimed inclaim 1, wherein the reinforcing fibers are selected from the groupconsisting of carbon fibers and aramide fibers.
 13. The composite asclaimed in claim 1, wherein the volume of the indicating fibers is atleast 2.5% of the total volume of the reinforcing fibers and theindicating fibers.
 14. A composite containing a matrix material of athermosetting plastic and a reinforcement layer, said reinforcementlayer comprising:a plurality of elongated reinforcing fibers, whereinsaid reinforcing fibers do not undergo a color change when wetted bysaid matrix material, said reinforcing fibers including at least a firstseries of fibers and a second series of fibers, said reinforcing fibersof said first series of fibers being generally aligned in a firstdirection, said reinforcing fibers of said second series of fibers beinggenerally aligned in a second direction at an angle to said firstdirection; a plurality of indicating fibers, said indicating fibersundergoing a color change when wetted by the matrix material, at leastone of said indicating fibers being generally aligned with saidreinforcing fibers of said first series of fibers, at least oneadditional indicating fiber of said indicating fibers being generallyaligned with said reinforcing fibers of said second series of fibers,said at least one and said at least one additional indicating fiberscrossing one above the other, the volume of said indicating fibers beingup to about 5% of the total volume of said reinforcing fibers and saidindicating fibers.
 15. The composite as claimed in claim 14, wherein thereinforcement layer includes a fabric, the first series of fibers is awarp, the second series of fibers is a weft, and the warp and weftinterweave to form the fabric.
 16. The composite as claimed in claim 14,wherein the reinforcement layer includes a mat, and each of the firstand second series of fibers is a mat ply.